The perfect gas equation, PV = nRT, is a fundamental tool in psychrometry as it allows us to manipulate the characteristics of air-water vapor mixtures. By rearranging the equation to p = dRT, we can analyze various conditions and definitions. The perfect gas equation is a simplification of real-world behavior, but it provides a useful approximation for many psychrometric applications, such as calculating the properties of air in air conditioning systems and cooling towers.